Many length measuring and distance measuring methods have been conventionally put into practice. In mechanical industry and electric industry, the length measurement and the distance measurement are performed for the purpose of measuring shapes and positions of precision apparatuses and precisely machined members. In the distance measurement and the length measurement, for example, an inspection is performed by measuring shapes and dimensions of precisely machined components before shipment, but this requires performing the measurement while making no contact with an object to be measured. Moreover, there have been demands on the precisely machined components for high measurement accuracy, typically accuracy of 0.1 mm or below. Furthermore, in a case where the object to be measured is large-scaled, as is the case with members of an electric generator or components of an aircraft, also required are conditions such as condition that a distance from a measuring instrument to the object to be measured needs to be increased. In addition, it is desirable that the measurement can be carried out in a work site while member machining and assembly, and this requires not only distance accuracy but also a method which is resistant to disturbance such as mechanical vibration and which has excellent measurement reproducibility and stability.
As the method of distance measurement and length measurement while making no contact with the object to be measured, an optical unit is suitable, and the measurement methods include: for example, a homodyne interference method, a heterodyne interference method, Time of flight (TOF), a Doppler method, and a triangulation method (Non-Patent Literature 1, Non-Patent Literature 2, and Patent Literature 1). Upon the length measurement and the distance measurement, required accuracy, a distance to the object to be measured, measurement time, and device configuration are considered to make selection from among these length measuring and distance measuring methods. For example, in a case where highly accurate distance measurement and shape measurement are required, widely used is a method of making distance measurement by extracting, by the homodyne interference method using a light interferometer, a phase change between light reflected or scattered from the object and light before irradiated to the object to be measured (Non-Patent Literature 1 and Non-Patent Literature 2). This method can achieve measurement with an accuracy of approximately one hundredth of a wavelength of used light, but due to reasons, for example, that the optical interference system is vulnerable to disturbance, a relatively large-scaled device is required, and that much time is required for measuring absolute distance to the object to be measured, applications in actual work sites in the mechanical industry and an electric factory are limited.
The Time of Flight is a method of by using pulse-like light, measuring absolute distance to the object to be measured based on a difference between time at which the light is emitted from a light source and time at which the light reflected or scattered at the object to be measured is detected by a photodetector. Due to simple principles, the distance measurement can be made in a relatively simple manner, but due to fast light propagation speed, there is limitation on frequency responsiveness of the photodetector and a circuit, and current measurement accuracy is approximately millimeters. It is not satisfactory for measuring the shapes and the positions of the precision and precisely machined members, and thus applications in work sites are limited.
The Doppler method is a method capable of measuring a moving speed and vibration of an object to be measured with an accuracy of approximately 0.05 μm/s based on a frequency change of the light reflected or scattered at the object to be measured. With the Doppler method, measured amounts are the moving speed and the vibration of the object to be measured, and the distance to the object to be measured is obtained by multiplying a speed of moving from a distance reference. A relative position from the distance reference can be measured with relatively high accuracy and device configuration is also relatively simple, and thus it is widely used in the mechanical industry and the electric industry. However, it is not suitable for measuring an absolute distance to the object to be measured and is also not suitable for the shape method since it is a method of obtaining a change in the relative distance from the reference distance by multiplying the moving speed.
The triangulation method is widely used in construction works, etc., but is less frequently used in fields of the mechanical industry and the electric industry for reasons, for example, that it can simply measure an absolute distance and a position but requires a large-scaled device for obtaining required measurement accuracy.
The tracking method is a method of measuring a shape of an object to be measured by use of the aforementioned distance measurement unit. In the tracking, a target is arranged on a surface of the object to be measured, light emitted from a light source is reflected on the target, and a distance to the target is measured by using returning light. It is a method of measuring the distance at different points of the target arranged on the surface of the object to be measured and then linking together these points to obtain a shape of the object to be measured. Since the target is arranged on the object to be measured, an amount of light returning to a photodetector can be increased and a distance from the light source to each target can be measured with high accuracy. However, since the target needs to be arranged on the surface of the object to be measured, preparation is very complicated. Moreover, since only the number of points of the target can be measured, the measurement points are spatially discrete, making it very difficult to recognize a small shape change of the object to be measured. Thus, it is used only in extremely limited fields of the mechanical industry and the electric industry.
As described above, in the work sites of the mechanical industry and the electric industry, it is difficult to measure the absolute distance to the object to be measured and the shape with high accuracy in actual work environment. However, on the other hand, for the purpose of achieving higher function of industrial products and their safety improvement, product sophistication and accuracy improvement are underway, and demands for measuring the absolute distance to the object to be measured and the shape thereof with high accuracy have been increased year after year.